The long range goal of the proposed research is to understand the molecular structure and physiological roles of DNA topoisomerase I (omega protein) from Escherichia coli and Salmonella typhimurium. The specific major objectives are: l. isolation and characterization of homogeneous omega protein from endonuclease-deficient mutants of S. typhimurium, 2. development of a radioimmunoassay to distinguish between the E. coli and S. typhimurium omega proteins, 3. genetic mapping of the omega protein structural gene, 4. isolation and characterization of omega mutants, 5. comparative studies of the E. coli and S. typhimurium omega proteins.